Approximate number sense is a shared evolutionary ability that underlies human's propensity for complex mathematics. Investigation of the neurobehavioral correlates of approximate number sense in healthy populations has helped elucidate how numeracy is represented in the brain, and has highlighted important behavioral signatures of abstract numerical discrimination that predict math performance throughout life (Halberda, Ly, Wilmer, Naiman, & Germine, 2012; Halberda, Mazzocco, & Feigenson, 2008; Libertus, Odic, & Halberda, 2012). The study of approximate number sense in patient populations with persistent neural and behavioral deficits in math and number processing, such as women with Turner syndrome (TS) (Hong, Scaletta Kent, & Kesler, 2009; Kesler, Menon, & Reiss, 2006; Molko et al., 2003), has the potential to highlight important components of numeracy processing that may improve our understanding of numerical cognition for all (Butterworth & Kovas, 2013). While our meta-analysis of behavioral performance on tests of math and number competency indicates basic numerical reasoning skills are largely preserved in this group (Baker & Reiss, 2015), very little is currently understood about the neural and behavioral correlates of approximate number sense in patient populations with genetic abnormalities leading to poor mathematics. The proposed K99/R00 research will directly address these gaps through a focused assessment of the neurobehavioral signatures of number sense in children, adolescents, and adult women with TS. This contribution will substantively advance our understanding of the effects of a specific genetic abnormality on domain-specific cognitive processes related to numerical cognition, and will help elucidate the underlying cause of poor math aptitude for the significant number of women living with TS. This contribution will accrue multiple tangible benefits to the scientific and TS communities alike. For example, findings from this project will encourage novel and more refined empirical hypotheses regarding the developmental progression of math deficits in patient populations with poor number sense. Furthermore, interrogation of the effectiveness of Web-based number sense training in this group will encourage the development of novel math interventions that are optimized for the specific cognitive constraints inherent in TS, but which may also be generalized to a broader population of individuals that struggle with mathematics. Finally, because the majority of research on numerical reasoning in TS has been conducted on children and teens, very little is understood about the neural and behavioral manifestation of these processes in adult women with TS. Thus, results from this project are expected to contribute new and fundamental information about numerical cognition throughout the life span of women with TS, and will directly advance the National Institute of Health and National Institute of Child Health and Development's overlapping mission to foster fundamental discoveries that provide all people with the chance to achieve their full potential for healthy and productive lives.